Global navigation satellite systems (GNSS), such as the American NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Defense Department is a satellite-based radio navigation system which transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with equal precision. The GPS uses 24 satellites dispersed in six, inclined, 12 hour circular orbits chosen to insure continuous 24 hour coverage world-wide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHZ and an L2 signal at a frequency of 1227.6 MHZ. The L1 and L2 signals are bi-phase modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigational data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN coded signal, a receiver decodes the signal to recover the navigational data, including ephemeris data. The ephemeris data is used in conjunction with a set of Kepler equations to precisely determine the location of each satellite. The receiver measures a phase difference (e.g., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of four equations to provide a space and time solution. The result is a precise determination of location of the receiver in three-dimensional space.
The velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured frequencies are used to determine Doppler frequency shifts for each satellite. The measured differences are used to solve another set of equations to determine a velocity of the receiver relative to the Earth based upon the detected phase shift of the received signals.
While the GPS performs relatively well, there are still situations where receivers cannot detect satellite signals. For instance, where a receiver is located in deep valleys or in the presence of interference, a receiver cannot detect a sufficient number of satellite signals to accurately determine position.
To overcome these difficulties, prior art systems have resorted to various signal enhancing techniques. For example, null steering may be used to reduce the effects of interferors. Beam forming may also be used to maximize signals where the position of a satellite is known.
The use of beam forming requires knowledge of the relative phase shift among the channels. Knowledge of the relative channel phase shift and gain may also be of value for null forming because it may permit determination of the direction of arrival of the interfering source. Accordingly, a need exists for a means of adapting the GPS receiver to changes in the antenna and circuit characteristics that affect the phase characteristics of an antenna.